1. Technical field
The present invention relates to a minisize breaker built into a secondary battery pack for an electrical equipment and the like.
2. Background Art
Heretofore, a breaker is employed as a protection device (safety circuit) for a secondary battery, electric motor and the like in various electrical equipments (see FIG. 8 and FIG. 9). If an abnormal situation happens, for example, the temperature of the secondary battery is excessively increased during discharging or being charged, or an overcurrent flows in an electric motor or the like provided in an equipment, e.g. automobile, home electric appliance and the like, then the breaker shuts off the current to protect the secondary battery, electric motor and the like. Such breaker used as a protection device is, in order to ensure the safety of the equipment, required to exactly function in response to temperature change (to have good temperature property) and also required to have a stable resistance value in the conduction state.
When a breaker is used as a protection device for a secondary battery and the like provided in an electrical equipment such as a notebook-size personal computer, a tablet type handheld terminal and a thin-shaped multifunctional mobile-phone called “smartphone”, miniaturization is required in addition to the above-mentioned ensuring of the safety of the equipment. In the case of the recent handheld terminals, in particular, the users' preferences toward miniaturization (thin-shaped) are strong. Equipments newly launched on the market by various manufacturers have a strong tendency to have miniaturized designs in order to obtain superiority in the design. Under such circumstances, a breaker—mounted together with a secondary battery, as one of component parts of a handheld terminal—is also strongly required to be further miniaturized.
The breaker is provided with a thermal actuator element operating in response to temperature changes so as to pass or shut off an electric current. In Patent document 1, a breaker in which a bimetal is used as a thermal actuator element is disclosed. A bimetal is formed by laminating two kinds of platy metal materials having different coefficients of thermal expansion. The bimetal is an element which changes its shape in response to temperature changes and controls conduction/nonconduction states of the contacts. The breaker disclosed in the document is composed of parts which are a fixed piece (base terminal), a movable piece (movable arm), a thermal actuator element, a PTC thermistor and the like and which are put in a resin casing. In order to use, the terminals of the fixed piece and the movable piece are connected to an electrical circuit of an electrical equipment.